Metal-air electrochemical power sources, particularly Al-air batteries and fuel cells with alkaline electrolyte are suitable for electric vehicles, unmanned aerial vehicles (UAV), reserve and emergency power supply and other applications.
Metal-air system with alkaline electrolyte has a great electrochemical capacity (about 8 k Wh/kg). However, during the operation of metal-Air batteries, products of the dissolution of metal anodes are accumulated in the electrolyte thus lowering the efficiency of the metal-air battery. Accordingly, following a certain operation time, the electrolyte solution needs to be replaced or regenerated.
One of the ways to regenerate the electrolyte is inducing the precipitation of the products of aluminum anode dissolution in the form of solid aluminum tri-hydroxide (ATH), and removing of ATH by filtration. However, the complete regeneration of alkaline solution by this method is not always possible. Accordingly, there is a need for method to remove the products of aluminum dissolution from alkaline electrolyte solutions, as complete as possible, and, preferably in a form of valuable byproducts.
Besides the above mentioned ATH, one group of desired byproducts that dissolved aluminum may be converted to is layered metal hydroxides (LMHs). LMHs consisting of positively charged metal hydroxide nano-layers and interlayer anions and water molecules in the interlayer. Layered metal hydroxides are used in the industry for ion-exchangers, catalyst, anti-acid, magnetic material, controlled release formulation, pharmaceutical products and polymer reinforcement materials, and transparent electrode and optoelectronic devices.
LMHs can be classified into three categories according to the structure and metal-ligand coordination type. One is layered double hydroxide (LDH) which can be represented by a general formula of [M2+1-xM3+x(OH)2]x+(An−)x/n.mH2O (M2+: divalent metal, M3+: trivalent metal, A: interlayer guest anion, 0<x<1, m and n are integers). Exemplified LDH is the naturally occurring mineral hydrotalcite (HTC) which is also referred to as magnesium aluminum hydroxy carbonate hydrate and is generally represented by the chemical formula: Mg6Al2(OH)16CO3.4H2O. HTC has found industrial usage in the following application areas.                Pharmaceuticals (antacid)        Polymer additive (acid scavenger, fire retardant)        Catalyst or catalyst support        Adsorbent        Ion exchange material        
Along with HTC described above, other LDH compounds exist and have been described in the technical literature. The defining characteristic of these materials is a layered structure where sheets of mixed metal hydroxides are separated by an interlayer gap region that contains water molecules and a negatively charged compound (anion) such as carbonate (CO3−2). A representation of this structure is shown in FIG. 12.
The negatively charged anions in the interlayer gap act as charge balancing sites to compensate for the positive charge induced by the presence of e.g. Al+3 atoms in a structure originally containing only Mg+2 atoms. Carbonate is the anion that occurs in the natural form of the HTC mineral. However, synthetic HTC can be prepared with any negative ion in the interlayer (OH1−, Cl1−, NO31−, SO42−, PO43− etc.). It is also possible to replace the primary cations (Mg and Al) with other metal ions of equal +2 or +3 charge such as Ca, Zn, Fe, Ni, La ions etc.
Another property of the LDH materials is that the interlayer anion can be replaced with an alternative anion via a process known as anion exchange. This allows a target anion such as arsenate (AsO4−3) or chromate (CrO4−2) to be incorporated into the HTC structure via exchange or absorption from an aqueous medium. This property formed the basis for the Alcoa Industrial Chemicals water treatment absorbent product Sorbplus®. All of the current industrial applications of HTC and LDH family compounds are based on synthetically produced material.